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. And
similarly no blood-vessel has in itself a separate individuality;
but they all form parts of one whole. For an isolated bone, if such
there were, would in the first place be unable to perform the office
for the sake of which bones exist; for, were it discontinuous and
separated from the rest by a gap, it would be perfectly unable to
produce either flexure or extension; nor only so, but it would
actually be injurious, acting like a thorn or an arrow lodged in the
flesh. Similarly if a vessel were isolated, and not continuous with
the vascular centre, it would be unable to retain the blood within
it in a proper state. For it is the warmth derived from this centre
that hinders the blood from coagulating; indeed the blood, when
withdrawn from its influence, becomes manifestly putrid. Now the
centre or origin of the blood-vessels is the heart, and the centre
or origin of the bones, in all animals that have bones, is what is
called the chine. With this all the other bones of the body are in
continuity; for it is the chine that holds together the whole length
of an animal and preserves its straightness. But since it is necessary
that the body of an animal shall bend during locomotion, this chine,
while it is one in virtue of the continuity of its parts, yet its
division into vertebrae is made to consist of many segments. It is
from this chine that the bones of the limbs, in such animals as have
these parts, proceed, and with it they are continuous, being
fastened together by the sinews where the limbs admit of flexure,
and having their extremities adapted to each other, either by the
one being hollowed and the other rounded, or by both being hollowed
and including between them a hucklebone, as a connecting bolt, so as
to allow of flexure and extension
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